In the papers "De Apologie van het denken"; "De moeizame gang van het denken"; "De bouw van de materie"; "Het wezen van de materie", the writer deals with the problem of matter as it presents itself to modern physics. His conclusion is: the physical world is to be considered as created by mind. It has no character apart from mind. All physical objects are built up of matter and matter itself -- as far as its nature can be grasped by the mental process of thinking -- is nothing else but the potential object created by the mind. It follows that physical science must necessarily express itself materialistically and mathematically. But the term "materialism" has lost the particular significance which it had some ten years ago: matter is not opposed to mind, but actually the outcome of a mind-process. On the other hand, what we really do when we are trying to understand the nature of matter, is, trying to understand the processes of thinking that lie behind. It is not denied that matter may have still another aspect, which remains hidden from the process of thinking, but which might be grasped by some other spiritual process. Physics, however, does not inquire into this question. The sole object of physical science is formed by the thinkable properties of matter. One could say that it busies itself only with dead matter. But as life cannot really know death, each attempt to picture the nature of matter is doomed to fail just in so far as it gives a picture. No picture can be adaequate. This is the fundamental reason why physics are impelled to break up matter and to reduce it to purely mathematical relations